(A reason I sometimes hear about is that "reply-guys" would somehow be discouraged from posting very popular replies if they saw how popular they are, i.e. that someone else has already posted the same thing elsewhere. I don't know whether this is true but if it is there are easier solutions, such as 1) fetch more replies when someone attempts to post a reply, 2) when a thread has a gazillion replies, give a warning "uh this thread is huge, maybe move it to a forum or take a walk in the park?".)